Endoplasmic reticulum (ER) is an organelle where protein synthesis takes place and that performs folding and carbohydrate modification of produced proteins as well as protein transport. Recently, stress on the endoplasmic reticulum is focused in relation, for example, with the onset of Alzheimer's disease (AD). Endoplasmic reticulum stress is considered to be induced by the inhibition of protein folding, carbohydrate modification, protein transport, and so on, that normally occur in the endoplasmic reticulum due to physiological imbalance, external factors, etc. For example, drugs such as tunicamycin and brefeldin A induce endoplasmic reticulum stress. Tunicamycin inhibits glycosylation of proteins in the endoplasmic reticulum, causing unfolded proteins to accumulate in the endoplasmic reticulum. Such accumulation of unfolded proteins causes induction of endoplasmic reticulum stress. Brefeldin A induces endoplasmic reticulum stress by causing accumulation of proteins through the inhibition of protein transport between the endoplasmic reticulum and Golgi apparatus.
The endoplasmic reticulum has stress response mechanisms against such endoplasmic reticulum stress. For example, a stress sensor, Ire, is known to recognize unfolded proteins and promotes protein folding by inducing the expression of molecular chaperones such as GRP78. However, when endoplasmic reticulum stress cannot be overcome by such stress response mechanisms, cells are indicated to die through apoptosis.
Clinical findings on endoplasmic reticulum stress have been reported as well. Analysis of genetic predisposition to familial Alzheimer's disease revealed mutation of preselenin-1 (PS1), a membrane protein that mainly exists on the endoplasmic reticulum. In addition to the promotion of beta-amyloid production that causes senile plaque formation and such, a characteristics of Alzheimer's disease, easy induction of apoptosis due to the increased sensitivity towards various apoptosis stimuli has been reported in cells expressing this mutant PS1 (Guo, Q. et al., J. Neurosci. 17:4212-4222 (1997); Guo, Q. et al., Nat. Med. 5:101-106 (1999)). Such cell death due to PS1 mutation has been shown to be suppressed via treatments that may reduce endoplasmic reticulum stress, for example, treatment with pharmaceutical agents and antioxidants that inhibit calcium release from endoplasmic reticulum.
On the other hand, DNA topoisomerase II is known to be degraded when stress is applied to human cancer cells. Under this situation, the cancer cells are known to show resistance to the anticancer agent VP-16 whose target is DNA topoisomerase II. At the same time, the expression of GRP78, a molecular chaperone, was also found to be increased (Yun, J. et al., Oncol. Res., 7:583-590, 1995). Furthermore, examination of the use of proteasome inhibitors on the stress response and drug resistance of cancer cells in human solid cancer revealed improved sensitivity toward anticancer agents due to suppressed stress response-induced degradation of topoisomerase II (Ogiso, Y. et al., Cancer Res., 60:2429-2434, 2000).
The response to endoplasmic reticulum stress is increased in solid cancer. This increased stress response is suggested to cause resistance against antitumor agents. The sensitivity toward anticancer agents of cancer cells is reported to increase upon inhibition of the induction of the above-mentioned molecular chaperones, such as GRP78 (Koomagi, R. et al., Anticancer Res. 19:4333-6 (1999); Fernandez, P. M. et al., Breast Cancer Res. Treat. 59:15-26 (2000); Katschinski, D. M. et al., J. Cancer Res. Clin. Oncol. 127:425-32 (2001)). Furthermore, GRP78 has been found to be an excellent target of anticancer agents (Jamora, C. et al., Proc. Natl. Acad. Sci. USA 93:7690-7694 (1996); Tomida, A. and Tsuruo, T., Anti-Cancer Drug Design, 14:169-177 (1999); Miyake, H. et al., Cancer Cells. J. Cell. Biochem., 77:396-408 (2000); Lee, A. S., Trends in Biochem. Sci., 26:504-510 (2001)).
Accordingly, the regulation of the mechanism of endoplasmic reticulum stress response by the expression of molecular chaperones such as GRP78 is important for the treatment of disorders like Alzheimer's disease, and to increase the sensitivity towards anticancer agents. Furthermore, substances that may regulate such endoplasmic reticulum stress response mechanism are useful for the development of therapeutic agents for disorders, such as Alzheimer's disease, and carcinostatics.